The present invention relates to power supply system, and more particularly, embodiments of the invention relate to a power supply system having a controller without having a feedback path from a secondary-side control circuit for controlling a pulse signal generator in a primarily control circuit.
There is an ongoing effort to replace conventional CCFL backlighting in large-screen LCD TVs with LED backlighting. LED backlighting provides many benefits. For example, the LED strings can be controlled separately instead of global dimming in a CCFL backlighting. Conventionally, a power conversion from an AC source to a DC source is required for the LED backlighting.
Some power converters use a diode rectifier followed by a bulk capacitor to convert an AC voltage to a DC voltage. These power converters often have a low power factor, which is defined as the ratio of the average power (consumed at the load) to the apparent power. One solution to improve the power factor is to add a filter for adjusting an AC source voltage to be in phase with a line current. The in-phase AC voltage is then applied to a resonant circuit that may include half bridge circuit having a pair of power switches to drive a resonant network.
Isolated AC to DC switched mode power supplies typically use a two-stage topology. The first stage serves the purpose of voltage transformation and isolation and outputs a sinusoidal current waveform with the help of a series resonance circuit to the primary winding of the isolation transformer. The second stage serves the purpose of providing a regulated output voltage and/or current to a load, e.g., a backplane lighting module. In general, a feedback circuit is interposed between the first stage and the second stage.
An example of a power supply system with a feedback circuit is shown in FIG. 1. A transformer T1 is interposed between a primary side circuit 110 and a secondary side circuit 120. Primary circuit 110 is coupled to a power source. An output of secondary circuit Vout is coupled to load circuit 130. A feedback circuit 125 provides a feedback signal related to the condition of the output on the secondary side and provides information regarding output condition to the control circuit at the first stage. The control circuit can use a pulse width modulation (PWM) control or a pulse frequency modulation (PFM) control. For example, the feedback circuit may generate an error voltage signal based on the actual voltage and the desired voltage at the output load.
In general, a PWM control circuit provides a large pulse width when the load is large, so that a constant output voltage can be maintained. A PFM control circuit maintains a constant voltage by maintaining a constant pulse width and but varying a pulse frequency, e.g., by increasing the pulse frequency when the load is large and by decreasing the pulse frequency when the load is small.
Although widely used, conventional power supply systems with feedback control tend to be complicated and expensive. Therefore, it is seen that more efficient and cost-effective techniques for improving power supply systems are highly desired.